This invention relates to a fuel cell system and more particularly to a combustor which heats a fuel processor which produces an H2-rich feed gas for consumption in a fuel cell stack.
Fuel cells have been used as a power source in many applications. For example, fuel cells have been proposed for use in electrical vehicular power plants to replace internal combustion engines. In proton exchange membrane (PEM) type fuel cells, hydrogen is supplied to the anode of the fuel cell and oxygen is supplied as the oxidant to the cathode. PEM fuel cells include a membrane electrode assembly (MEA) comprising a thin, proton transmissive, non-electrically conductive solid polymer electrolyte membrane having the anode catalyst on one of its faces and the cathode catalyst on the opposite face. The MEA is sandwiched between a pair of electrically conductive elements which (1) serve as current collectors for the anode and cathode, and (2) contain appropriate channels and/or openings therein for distributing the fuel cells gaseous reactants over the surfaces of the respective anode and cathode catalysts. The term fuel cell is typically used to refer to either a single cell or a plurality of cells (stack) depending on the context. A plurality of individual cells are commonly bundled together to form a fuel cell stack and are commonly arranged in series. Each cell within the stack comprises the membrane electrode assembly (MEA) described earlier, and each such MEA provides its increment of voltage. A group of adjacent cells within the stack is referred to as a cluster. Typical arrangements of multiple cells in a stack are described in U.S. Pat. No. 5,763,113, assigned to General Motors Corporation.
In PEM fuel cells, hydrogen (H2) is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can be either a pure form (O2), or air (a mixture of O2 and N2). The solid polymer electrolytes are typically made from ion exchange resins such as perfluoronated sulfonic acid. The anode/cathode typically comprises finely divided catalytic particles, which are often supported on carbon particles, and mixed with a proton conductive resin. The catalytic particles are typically costly precious metal particles. These membrane electrode assemblies are relatively expensive to manufacture and require certain conditions, including proper water management and humidification, and control of catalyst fouling constituents such as carbon monoxide (CO), for effective operation.
For vehicular applications, it is desirable to use a liquid fuel such as an alcohol (e.g., methanol or ethanol), or hydrocarbons (e.g., gasoline) as the source of hydrogen for the fuel cell. Such liquid fuels for the vehicle are easy to store onboard and there is a nationwide infrastructure for supplying liquid fuels. However, such fuels must be dissociated to release the hydrogen content thereof for fueling the fuel cell. The dissociation reaction is accomplished within a chemical fuel processor or reformer. The fuel processor contains one or more reactors wherein the fuel reacts with steam and sometimes air, to yield a reformate gas comprising primarily hydrogen and carbon dioxide. For example, in the steam methanol reformation process, methanol and water (as steam) are ideally reacted to generate hydrogen and carbon dioxide. In reality, carbon monoxide and water are also produced. In a gasoline reformation process, steam, air and gasoline are reacted in a fuel processor which contains two sections. One is primarily a partial oxidation reactor (POX) and the other is primarily a steam reformer (SR). The fuel processor produces hydrogen, carbon dioxide, carbon monoxide and water. Downstream reactors may include a water/gas shift (WGS) and preferential oxidizer (PROX) reactors. In the PROX, carbon dioxide (CO2) is produced from carbon monoxide (CO) using oxygen from air as an oxidant. Here, control of air feed is important to selectively oxidize CO to CO2. A combustor typically is included in a fuel cell system and is used to heat various parts of the fuel processor, including reactors, as needed.
Fuel cell systems which process a hydrocarbon fuel to produce a hydrogen-rich reformate for consumption by PEM fuel cells are known and are described in U.S. patent application Ser. Nos. 08/975,422, which corresponds to U.S. Pat. No. 6,232,005 issued on May 15, 2001, in U.S. Ser. No. 08/980,087, which corresponds to U.S. Pat. No. 6,077,620 issued on Jun. 20, 2000, and in U.S. Ser. No. 09/187,125, which corresponds to U.S. Pat. No. 6,238,815 issued on May 29, 2001, each of which is assigned to General Motors Corporation, assignee of the present invention; and in International Application Publication Number WO 98/08771, published Mar. 5, 1998. A typical PEM fuel cell and its membrane electrode assembly (MEA) are described in U.S. Pat. Nos. 5,272,017 and 5,316,871, issued respectively Dec. 21, 1993 and May 31, 1994, and assigned to General Motors Corporation.
Efficient operation of a fuel cell system depends on the ability to effectively control operation of major interdependent components or subsystems such as the combustor and fuel processor. The interpendent operation of the combustor and fuel processor render control of each particularly difficult. The combustor heats up the fuel processor to a temperature sufficient for the fuel processor to generate hydrogen-rich feed for the fuel cell. Then, the combustor is at least partially fueled by the hydrogen-rich stream from the fuel processor.
Therefore, it is desirable to provide a method to determine whether a nominal expected relationship exists between certain combustor and fuel processor operating parameters.
The present invention is directed to the operation of a fuel cell system which comprises a combustor which heats a fuel processor which, in turn, generates a hydrogen-rich feed stream for use in a fuel cell stack. The hydrogen-rich feed stream is consumed in the fuel cell stack whereby electricity is produced. The present invention provides a new method for operating the combustor within the system and, particularly, an improved method for regulating fuel input to the combustor. In one aspect, supplemental, hydrocarbon fuel input to the combustor is regulated based on the temperature of the fuel processor. Preferably, the fuel processor comprises a reactor having a catalytic bed, and the fuel input to the combustor is adjusted based on a change in temperature of such catalytic bed.
In one aspect, the invention provides a method for operating a combustor in response to a monitored of the fuel processor, detecting a relatively low temperature of the fuel processor, adjusting hydrocarbon fuel supply to the combustor, and comparing such adjusted supply (fuel flow) to the combustor to a predetermined fuel flow rate or range of fuel flow rates. In order to further appreciate features of the invention, it is helpful to understand the relationship between the combustor, fuel processor, and fuel cell stack.
The fuel processor generates a hydrogen-rich product (feed stream) from a hydrocarbon. The hydrogen-rich feed stream from the fuel processor is supplied to a fuel cell stack which generates electricity by oxidation of the hydrogen with oxygen. In a preferred start-up mode, a hydrocarbon fuel stream and an air stream are supplied to the combustor. The hydrocarbon fuel and air are reacted or burned in the combustor in order to generate heat to heat the fuel processor. The products of the combustion reaction in the combustor are supplied to the fuel processor.
Preferably, one or more reactors within the fuel processor are heated by indirect heat transfer from the products of combustion. After the products of combustion from the combustor have begun to heat the fuel processor, a hydrocarbon reactant is supplied to the fuel The hydrocarbon reactant is reacted with steam, air, or a combination of both in the fuel processor. The reaction between the hydrocarbon reactant and the steam and/or air produces a hydrogen-rich feed stream which is usable in the fuel cell stack to produce electricity.
In one aspect, after the fuel processor has attained and maintained its desired temperature, it produces the hydrogen-rich stream which is consumed in the fuel cell stack to produce electricity. However, the quantity of hydrogen supplied to the fuel cell stack is greater than that required to produce the increment of power desired from the system, therefore, at least a portion of the hydrogen-rich feed stream is not consumed in the fuel cell stack and is directed to the combustor. This excess portion of the hydrogen-rich feed stream is reacted with the air stream in the combustor for generation of heat to heat the fuel processor.
Therefore, the invention provides a method of operating a combustor to heat a fuel processor in a fuel cell system, in which the fuel processor generates a hydrogen-rich stream, a portion of which is consumed in a fuel cell stack and a portion of which is discharged from the fuel cell stack and supplied to the combustor. Accordingly, first and second streams are supplied to the combustor. The first stream is a hydrocarbon fuel stream and the second stream consists of the hydrogen-rich stream. More particularly, the method comprises the steps of: monitoring the temperature of the fuel processor; regulating the quantity of the first stream to the combustor according to the temperature of the fuel processor; and comparing said quantity of said first stream to a predetermined value or range of predetermined values.
In another aspect, the method of the invention includes generating an output signal when the quantity of the fuel stream supplied to the combustor is different from a predetermined value. Preferably, this comparison occurs after each adjustment in the quantity takes place. Such comparison preferably occurs before and/or after each adjustment. If desired, the quantity of the first stream may be compared to a range of predetermined values and an output signal generated if the value is outside the predetermined range.
A variety of corrective actions are possible once it is determined that the quantity of the first stream is different from a predetermined value or range of predetermined values.
In a preferred aspect, the method further includes increasing the quantity of the first stream to the combustor in response to a decrease in the temperature of the fuel processor.
Another corrective action is terminating operation of the fuel cell stack when the output signal is generated. It is possible to block the output signal for a period of time called a time delay and then terminate operation of the fuel cell stack when the time duration of the signal exceeds a predetermined time period.
In a preferred aspect, the fuel processor comprises a reactor having a catalytic bed and the monitored temperature of the fuel processor is the temperature of such bed. By monitoring the temperature of the catalytic bed in the fuel processor, a determination is made whether to-adjust the supply of the first stream to the combustor in accordance with such monitored temperature.
In summary, in a preferred aspect, the method of the invention determines a relatively low temperature level in the fuel processor, increases the quantity of the first stream supplied to the combustor, and then compares such adjusted quantity to a desired or predetermined value or range of values.